The early 2000s heralded a reversal of long-term trends for declining grain prices (the first such switch since the 1960s); this coincided with greater expansion of agricultural land into formerly 'natural' areas.

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Projections of future food production have often relied on the assumption that yields will increase for major crops. However, given the evidence that yields in certain areas have plateaued over recent years, challenges such as growing resistance to agricultural chemicals and the threats of water scarcity and climate change, this may not be the case and experts suggest that yields in more areas may begin to plateau or even decline over the next century.

Arguing that measures to ensure food security must rely on the accurate assessment of yield gains, Agronomists and statisticians from the University of Nebraska have come together to assess yield predictions.  They warned that climate change mitigation requires humans to reign in damaging land-use change, which is already thought to be the source of between 15 and 25 percent of anthropogenic greenhouse gas emissions worldwide (largely through the conversion of carbon-rich and biodiverse ecosystems, which take carbon from the atmosphere, into monocrop agricultural production, an emitter), but that current assessments do not adequately address the seriousness of this trend.

In a report published on Tuesday (17^th December) in the journal Nature Communications, the researchers suggested that "Previous projections of food security are often more optimistic than… historical yield trends would support." They said that the expansion of croplands into new areas – and the pollution this causes – may be temporary or permanent, depending on food prices and long-term trends in crop yields.

Through their research, Professor Kenneth Cassman and colleagues said estimations of future global food production and its ability to meet the dietary needs of a growing human population have mostly been based on projections made from historical trends. They pointed out that past trends have been dominated by the rapid adoption of new technologies – sometimes one-time innovations – which allowed for an increase in crop production, whereas currently available information suggests this is no longer the case.

The researchers added that, whilst some global regions are not yet fulfilling their yield potential, croplands in these areas have expanded rapidly nonetheless, and in certain other areas evidence suggests abrupt decreases could cut yield gains. Areas at risk of decreases include eastern Asia, where rice production could suffer, and northwest Europe, where wheat production is set to be hit. Both areas are massive global suppliers of these crops (wheat and rice).

In Britain, which lies in the Western European area, climate projections suggest that more rain will fall in more intense bursts, though less often. This could, in the absence of adequate adaptation measures, lead to periods of drought and flooding.

The Nebraskan scientists sought to create a more accurate picture of future food production, using more available information, statistical analysis and historical trends. Their assessment yielded some concerning results.

30 percent of cereal crops may have reached 'peak yield'

According to the scientists, around 30 percent of the main global cereal crops, including rice and wheat, may have reached their maximum possible crop yield potential in farmer's fields. Yields of these crops have recently displayed an abrupt decrease or have plateaued and, looking to the future, the researchers questioned predictions which are "typically based upon a constant increase in yield; a trend that this research now suggests may not be possible."

Cassman and colleagues examined past yield trends for cereal, oil, sugar, fibre, pulses, tuber, root crops, rice, wheat and maize, in countries with the highest levels of cereal production and found evidence does not support projections of crop yield increases in many cases. Rather, their data suggest that the rate of yield gain has recently decreased or stopped for one or more of the major cereals in many of the most intensively cropped areas of the world, including the aforementioned areas of Asia and Europe, as well as the United States.  

The forecast for maize is exceptionally bleak. The scientists said that, whereas the stagnation in yield gain affects around 33 percent of the global rice and 27 percent of wheat production (largely from Asia and Europe respectively), in China the increase in crop yields in wheat has remained constant, but maize yield increase has slowed by 64 percent for the period 2010-2011 compared to ten years previous. This decrease has occurred despite an increase in investment in agricultural research and development, education, and infrastructure. This, they said, suggests that in many areas a maximum potential of yield production may have been reached.

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The researchers said that, where yield increases have occurred, this has largely been associated with crop production moving into new areas. Their research showed that a period of global yield increases came to an end in 2002, and that since this time increases have been driven by the expansion of agricultural land (mostly in Africa, Asia and South America). According to the team, the global area under crop has increased by almost ten million hectares per year (an area larger than the whole of Scotland), and 60 percent of the newly cropped land is being used to cultivate the major cereals.

Professor Cassman and his colleagues said that accurate estimates of yield trends are essential to inform policy makers, who need to develop strategies and fund research to achieve future global food security.  They said that sustaining further yield gains under the current paradigm would likely require the fine tuning of many different factors in the production of crops, but added that "the associated marginal costs, labour requirements, risks, and environmental impacts may outweigh the benefits."

What is clear from the scientists' assessment is that the current pattern of land-use change cannot continue; further destruction of the world's carbon sinks would lock humanity into a pattern of more severe pollution and climate change, which would result in further severe decreases of yields in susceptible areas.

The study is available to read here